MACKIE — FIRST TRACES OF THE SUCCESSION OF LIFE. 



387 



mostraca) abound ; fisli next appear in number ; tlien reptiles reign ; 

 tlien dawns the era of the gigantic mammals ; and tlien tbe Age of 

 Man sets in. So in vegetation, as far as we can judge from tbe 

 strange and singular fossil-forms presented to our view, tlie flower- 

 less preceded the flowering plants and trees which so luxuriantly 

 covered the Tertiary lands, and still adorn our own. 



But this remarkable advance, so evident when we regard the 

 gTander groups and the results of ages as a whole, becomes less 

 apparent and indeed very obscure when we attempt to combine the 

 seeming links of minor details and to trace one form developing 

 itself into another. We see the age of reptiles succeed that of fish ; 

 the ao'e of man followino- on that of the lower mammals ; and we 

 have no difficulty in appreciating the higher stages of each, and the 

 successively improved conditions of our planet to which they were 

 adapted; but when we attempt to trace out links to join the reptile 

 with the fish, and the quadruped mammal with the man, we fail. 

 We may see resemblances of development on either hand, but no 

 true junction-forms ; and then again when, as between genera and 

 families, we do meet with connecting species, such may occur in time 

 either in advance or in arrear of the ao;e or ao^es in which the forms 

 so connected existed ; while, on the other hand, some genera, such 

 for example as the Lingula we meet mth for the first time in the 

 Lower Silurian rocks, have lasted from their first appearance to the 

 present hour with scarcely more than a specific difference between 

 those primitive individuals and those now living in our seas. One 

 thing, however, seems certain of the Lower Silurian moUusca, Crus- 

 tacea, and annelides, that their geographical distribution was far 

 greater and much more universal than is the case with existing 

 species ; and although lines of demarcation have been attempted to 

 be drawn between many of the American Lower Silurian fossils and 

 those of our own country, it is very questionable, at least in some 

 cases, if any real specific distinctions exist. 



The general appearance and character of the Stiper-stones have 

 been well described by Sir Roderick Murchison, in whose wake, so 

 great has been his own energy and so powerful the means at his dis- 

 posal, a British author writing on Silurian strata, is almost com- 

 pelled to follow. " Trending in a broken mural line from north- 



